poly_area.c running 
data x1= 0.00000,  y1= 5.00000 
data x2= 1.12257,  y2= 1.54508 
dx=1.12257, da=115.931, area=115.931 
data x2= 4.75528,  y2= 1.54508 
dx=3.63271, da=368.884, area=484.814 
data x2= 1.81636,  y2=-0.59017 
dx=-2.93892, da=-295.295, area=189.519 
data x2= 2.93893,  y2=-4.04508 
dx=1.12257, da=109.655, area=299.175 
data x2=-0.00000,  y2=-1.90983 
dx=-2.93893, da=-285.142, area=14.0321 
data x2=-2.93893,  y2=-4.04508 
dx=-2.93893, da=-285.142, area=-271.11 
data x2=-1.81636,  y2=-0.59017 
dx=1.12257, da=109.655, area=-161.455 
data x2=-4.75528,  y2= 1.54508 
dx=-2.93892, da=-295.295, area=-456.75 
data x2=-1.12257,  y2= 1.54508 
dx=3.63271, da=368.884, area=-87.8664 
data x2= 0.00000,  y2= 5.00000 
dx=1.12257, da=115.931, area=28.0643